As distributed generation (DG) systems become part of the power grid, there is an increased safety hazard caused by islanding for personnel and more risk of damage to the equipment. Islanding refers to the condition in which a distributed generator (DG) continues to power a segment of a distribution network or grid even though electrical grid power from the electric utility is no longer present. As shown in FIGS. 1A and 1B, the islanding phenomenon happens when the grid is intentionally or accidentally disconnected from the network and the DG continues to energize local loads. FIG. 1A shows a grid network 100 before islanding occurs and FIG. 1B shows a grid network 100 after islanding occurs. As seen from FIG. 1B, when an islanding condition exists, the main power system 102, which may be a power source provided by a utility company, an electricity cooperative, permanent or semi-permanent generation, etc., is disconnected from the rest of the grid network 100 by the dis-connection 102, whereby, the DG units 106 will feed the load 108 unless the DG units cease to generate power.
Aside from the danger to maintenance personnel arriving to service a circuit, also called a feeder, that is energized by DG systems, there are also operational issues due to islanding. The voltage and frequency may not be maintained within the range of IEEE 1547-2003 standard. This standard stipulates a maximum delay of 2 seconds for detection on an unintentional island and all DG systems are required to cease energizing the load network, which may be a power grid. Typically, the islanded system may also be insufficiently grounded by the interconnection inside the DG. Reclosure operations that are initiated by the utility to clear the fault may also cause large mechanical torques, along with currents, particularly at in-rush, which are harmful for equipment in the islanded network.
A common example of islanding may occur at a grid supply line that has solar panels attached to it. In the case of a blackout, the solar panels will continue to deliver power as long as there is sufficient sunlight. In this case, the supply line becomes an “island” with power surrounded by a “sea” of unpowered lines. For this reason, solar inverters that are designed to supply power to the grid are generally required to have some sort of automatic anti-islanding circuitry in them.
Islanding detection methods can be classified into two major groups: remote and local methods. Remote techniques are based on the communication between utilities and DG systems such as power line communication, and supervisory control and data acquisition that do not have non-detection zone (NDZ), but are expensive to be implemented and therefore uneconomical. NDZs are defined as a loading condition for which an islanding detection method is unable to detect islanding. Local techniques, which are just related to the DG, can be classified into two major categories: passive and active methods. Passive methods are based on measuring local parameters of DG and comparing the parameters to a reference value. Some commonly applied passive methods are over/under frequency protection (OFP/UFP), over/under voltage protection (OVP/UVP), phase jump detection, voltage harmonic monitoring and change in grid impedance detection. While these methods are simple to implement, typically, they fail to detect islanding in one or more powering/loading condition(s) leading to NDZ(s) for these methods. NDZs exist for OVP/UVP or OFP/UFP methods when the inverter generated power closely matches that of the load and, for the phase jump detection method when the load power factor is unity.
Active methods strive to reduce the NDZs associated with typical passive methods by adding field quantities, such as voltage, current, perturbations to the inverter. Some active methods include: (i) Output power variation method requires multiple DGs but it fails when synchronization is not met due to the averaging effect; (ii) Active frequency drift (AFD) method requires adding small increments/decrements in the frequency of the inverter output current while monitoring the frequency of the voltage. AFD fails to detect an islanding condition when the load phase angle matches the phase offset of the perturbation. Sandia frequency shift (SFS) method which is an active frequency adjustment improves the performance of the AFD method by adding positive feedback to adjust the frequency away from the nominal value faster than the AFD method. Potential islanding conditions may be detected by the SFS method when the frequency traverses out of the acceptable range. However it may also fail as the phase angle of the load depends on the operating frequency. Accordingly, there is a need for systems and methods that are cost efficient and effective at detecting whether an Islanding condition exists.